The usefulness and safety of nuclear power generation, problems of radioactive contamination and the like have received attention particularly after the earthquake disaster. Effective utilization of renewable energy is essential for establishing a sustainable society which does not depend on fossil resources while reducing emissions of carbon dioxide in thermal power generation. Particularly, utilization of solar energy, which is the most dominant in renewable energy, is very important, but the utilization techniques of solar energy are limited. Examples of the technique that utilizes solar energy include solar photovoltaic power generation, solar heat, biomass and artificial photosynthesis.
Hydrogen can be converted into clean energy that does not emit carbon dioxide. Hydrogen reacts with oxygen to generate electric power, heat and water. Hydrogen produces energy with high efficiency, and is used in a wide range of applications including fuels for automobiles and power generation equipment. A solar light hydrogen production technique for producing hydrogen and oxygen by directly decomposing water using a photocatalyst and a photoelectrode including an oxide semiconductor is inexpensive, and studies on the solar light hydrogen production technique are actively promoted for realization of a future hydrogen energy-based society.
In the field of photocatalysts, various photocatalyst materials (oxide semiconductors) are studied. Titanium oxide is inexpensive, and the application of titanium oxide to a solar energy conversion material as a material of a photoelectrode and a photocatalyst receives attention. A TiO2 photocatalytic reaction has such a characteristic that light (ultraviolet light) having a wavelength of 350 nm is effective for the reaction. However, solar light includes only a small amount of ultraviolet light, and for efficient energy conversion, it is absolutely necessary to utilize visible light, which contains half of solar energy. Studies for modifying titanium oxide to impart visible light responsiveness thereto are extensively conducted, and studies for reforming a metal surface are also conducted (see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2006-255865